It is known to prepare hydrocarbon fractions such as naphtha, kerosene and gas oil by means of two-stage hydrocracking of heavy hydrocarbon oils, such as vacuum distillates of crude mineral oil. To this end a heavy hydrocarbon oil will be hydrocracked in a first stage reaction section, the hydrocracked effluent will be passed through a heat exchanger to pre-heat the feedstock for the first stage, and the hydrocracked effluent will subsequently be passed through a furnace to reheat the hydrocarbon effluent before entering the distillation unit. In the distillation unit the hydrocracked effluent from the first stage reaction section will be separated by distillation into one or more light hydrocarbon fractions and a heavy hydrocarbon fraction. The heavy hydrocarbon fraction will be fed to a second stage hydrocracking reaction section. Before entering the second stage the heavy hydrocarbon fraction from the distillation unit will be heated by means of a heat exchanger using heat of the hydrocracked effluent from the second stage. The hydrocracked effluent obtained from the second stage reaction section will also be reheated by means of a furnace, and separated by distillation into one or more light hydrocarbon fractions and a heavy hydrocarbon fraction. The heavy hydrocarbon fraction so obtained from the hydrocracked effluent from the second stage will be recycled from the distillation unit to the second stage, whereas light hydrocarbon fractions separated in the distillation are recovered as the desired end products. The hydrocracked product streams from the first and second reaction stages will usually both have a temperature of about 250° C.
A drawback of such a known two-stage hydrocracking process is the build up of so-called polyaromatic compounds (PCAs) in the recycle stream from the distillation unit to the second stage when very high conversion levels are applied in the processing of heavy feedstocks. At high levels such polyaromatic compounds will deposit in the recycle circuit as a result of which equipment such as heat exchangers will become blocked and the energy efficiency of the two-stage hydrocracking process will be seriously affected.
In order to deal with this problem of PCAs build up various approaches have been suggested. It has for example been proposed to apply a substantial bleed stream of the heavy recycle product which contains PCAs. However, this reduces the overall conversion of the feedstock and results in a reduced yield of valuable lighter products such as high quality kerosene, and high quality gas oil.
In U.S. Pat. No. 4,961,839 a hydrocracking process is described, wherein a method is disclosed for counteracting the formation and accumulation of polycyclic aromatic compounds within a hydrocracking process unit. The bottoms fraction comprising the PCA's is removed. The disadvantage of the process as described in U.S. Pat. No. 4,961,839 is that the overall conversion of the feedstock is reduced and results in a reduced yield of valuable lighter products such as high quality kerosene, and high quality gas oil.
U.S. Pat. No. 5,120,426 describes an improved hydrocracking process, wherein the feed comprises foulant. The bottoms of the hydrocracker is being cooled in heat exchangers. The foulants are being removed via cooling and filtering. A disadvantage of the process is that these filters need to be replaced.
U.S. Pat. No. 6,858,128 describes a catalytic hydrocracking process with one hydrocracking reactor. The stream comprising an enhanced level of heavy polynuclear aromatic compounds is removed from the divided wall fractionator.
In addition to the above, it has been suggested to adsorb PCAs from the recycle stream by using activated coal. Such an approach, however, requires additional equipment, whereas the active coal loaded with the carcinogenic PCAs needs subsequently to be disposed of.
Thus, there is a need for a multi-stage hydrocracking process wherein the above disadvantage of PCAs build up is sufficiently dealt with in an attractive economic manner.